The present invention relates to a technology of controlling opening and closing timing of an intake valve and an exhaust valve in an internal combustion engine, especially to a technology of improving engine startability at a low temperature engine start.
There is a technology, for example, disclosed in Japanese Unexamined Patent Publication No. 2000-73797, for controlling opening and closing timing of an intake valve and an exhaust valve in order to improve engine startability.
In this technology, at an engine start time, a valve overlap of the intake valve and the exhaust valve is expanded with respect to valve operation characteristics at a low load and low rotation of an engine to be set by a normal control, to improve a start-up response of an engine rotation (cranking speed) at the engine start time by decreasing intake and exhaust loads, and then the valve control is returned to that in a normal valve timing when the engine rotation speed reaches a predetermined value or above.
In starting the engine at a low temperature, however, there is a case where the engine rotation speed is not increased due to a lack of a combustion pressure in initial explosion. In such a case, according to the above conventional technology, since the valve overlap continues to be expanded during the combustion after the initial explosion, a fluctuation in the combustion pressure becomes large due to an exhaust gas (internal EGR) taken into an intake air at a valve overlapped period, thereby bringing a misfire, an engine stall at the worse, and then a failure of engine start.
The present invention, in view of the forgoing problem, has been achieved and an object of the present invention is to improve engine startability at a low temperature engine start.
In order to achieve the above object, with the present invention, an intake valve and an exhaust valve are controlled as follows.
A temperature and starting of an engine are detected, and if a low temperature engine start at a predetermined temperature or lower is detected, a valve overlap amount in which the opening timing of the intake valve and the opening timing of the exhaust valve are overlapped with each other is set to be equal to or less than a valve overlap amount in reference valve operation characteristics to be normally set, while controlling the closing timing of the intake valve to the vicinity of the intake bottom dead center with respect to the reference valve characteristics.
By controlling the closing timing of the intake valve to the vicinity of the intake bottom dead center, an air amount (air charging efficiency) taken into the engine can be increased, so that a lack of combustion pressure in initial explosion at a low temperature engine start can be made up to ensure good engine startability. At the same time, since the valve overlap amount is set to be equal to or less than the valve overlap amount in the reference valve operation characteristics to be normally set at the engine start, a deterioration of the combustion performance due to an increase of an exhaust gas amount (internal EGR) taken into an intake air at a valve overlapped period after the initial explosion can be avoided.
The other objects and features of this invention will become understood from the following description with accompanying drawings.